A computed tomography (CT) scanner includes a rotating portion rotatably supported by a stationary portion. The rotating portion supports an x-ray tube, which emits radiation that traverses an examination region and a portion of an object or subject therein, and a detector array that detects radiation traversing the examination region and generates projection data indicative of the detected radiation. A reconstructor reconstructs the projection data and generates volumetric image data indicative of the portion of the object or subject in the examination region. One or more images can be generated based on the image data.
The detector array includes detector pixels that convert detected x-ray photons into electrical signals indicative thereof. For each revolution of the rotating gantry, the detector pixels detect and convert x-ray photons for a plurality of integration periods, each corresponding to a different angular position range. The time duration of an integration period depends on the rotating gantry rotation speed and the number integration periods for each revolution of the scan. With an integrating detector array, at the beginning of each integration period, the integrators for the detector pixels are reset, and then the integrators receive and integrate the electrical signals over the integration period. The integrated signals form the projection data for that integration period.
The reconstructor processes projection data for a given integration period as corresponding to a particular angular position range. However, if the integration period timing with respect to the angular position range is not accurate, then, in actuality, the projection data corresponds to a different angular position range, and this may result in degraded image quality of the reconstructed image data. Unfortunately, there is a natural rotating gantry position error in which the dominant disturbance of the rotating gantry velocity is imbalance, which is a function of rotating gantry angle, and the amount of angular error due to the imbalance is a function of the inverse of the cube of the rotation speed.
A highly accurate large diameter rotating gantry position determining apparatus has been used to determine the actual position of the rotating gantry, which has been used to synchronize the integration period timing and rotating gantry angular position range. Such an apparatus has included a high resolution encoder and resolver, supporting electronics, a power supply, and a slip ring to transfer data between the rotating and stationary portions of the scanner. Unfortunately, such apparatuses generally are complex and expensive, and increase the overall cost of the scanner.